Rachel Shoaf appeared Wednesday in Monongalia County Circuit Court after agreeing to plead as an adult, said U.S. Attorney William Ihlenfeld. She is in custody until sentencing.

The plea agreement offers no insight into the motive for the slaying but says Shoaf inflicted the fatal wounds on 16-year-old Skylar Neese, an honors student at University High school.

Neese's remains were found in Wayne Township, Pa., in January, about 30 miles from her family's home.

Prosecutors plan to recommend a 20-year sentence for Shoaf and indicate they will oppose any move to have her sentenced as a juvenile.

Shoaf's family issued a statement through Morgantown attorney David Straface, apologizing to Neese's family and saying they were shocked to learn of their daughter's involvement.

"There is no way to describe the pain that we, too, are feeling," they said. "We are truly sorry for the pain that she has caused the Neese family, and we know her actions are unforgiveable and inexcusable. Our daughter has admitted her involvement and she has accepted responsibility for her actions.

"Our hearts are broken for your loss, and we are still trying to come to terms with this event," they said. "We pray that we all will find peace in our hearts and the strength to move forward."

Shoaf's attorney declined further comment.

U.S. Attorney William Ihlenfeld said a second teenager was in custody, but authorities haven't named her or said what she's being charged with.

Straface said he is not representing that suspect, and the Monongalia County prosecutor's office didn't immediately return messages.

Neese was last seen on surveillance video leaving her family's apartment voluntarily and getting into a car on July 6, 2012. She was initially considered a runaway, but her parents soon suspected she was abducted.

The recovery of her body led to changes in West Virginia's Amber Alert process.Legislators recently passed "Skylar's Law" so Amber Alerts are not limited to kidnappings. It now requires law enforcement officials to relay initial reports of any missing child to State Police, who then contact the Amber Alert system.

Amber Alert personnel would then decide whether to issue an alert.

University High School, Principal Shari Burgess would not confirm that Shoaf was a student there but said that anytime a tragedy occurs, counselors are on hand to help students.

Though classes went on uninterrupted Wednesday, Burgess said there would be an announcement advising students that help was available for those who need it.